We are examining which porphyrins are bound by hemopexin and may be catabolized with the aid of this protein. We showed that the catabolism of hemopexin is increased whether heme is administered as heme, hemoglobin or methemoglobin. Photo-inactivation and chemical modification of the purified protein rendered a molecule which lost heme-binding in vitro and in vivo. With an isolated rat liver perfusion system, we found the rate of conversion of heme into bilirubin to be comparable to that reported for in vivo studies. We encountered a reduced production of bilirubin when the animals were treated with porphyrinogenic agents.